diff --git a/packages/flutter_tools/lib/src/asset.dart b/packages/flutter_tools/lib/src/asset.dart
index c03d5f44cd..c5fa45ab28 100644
--- a/packages/flutter_tools/lib/src/asset.dart
+++ b/packages/flutter_tools/lib/src/asset.dart
@@ -37,8 +37,8 @@ const String kFontManifestJson = 'FontManifest.json';
 const List<Map<String, Object>> kMaterialFonts = <Map<String, Object>>[
   <String, Object>{
     'family': 'MaterialIcons',
-    'fonts': <Map<String, Object>>[
-      <String, Object>{
+    'fonts': <Map<String, String>>[
+      <String, String>{
         'asset': 'fonts/MaterialIcons-Regular.otf',
       },
     ],
@@ -161,7 +161,8 @@ class ManifestAssetBundle implements AssetBundle {
 
   @override
   bool needsBuild({ String manifestPath = defaultManifestPath }) {
-    if (_lastBuildTimestamp == null) {
+    final DateTime lastBuildTimestamp = _lastBuildTimestamp;
+    if (lastBuildTimestamp == null) {
       return true;
     }
 
@@ -179,13 +180,13 @@ class ManifestAssetBundle implements AssetBundle {
         if (dateTime == null) {
           continue;
         }
-        if (dateTime.isAfter(_lastBuildTimestamp)) {
+        if (dateTime.isAfter(lastBuildTimestamp)) {
           return true;
         }
       }
     }
 
-    return stat.modified.isAfter(_lastBuildTimestamp);
+    return stat.modified.isAfter(lastBuildTimestamp);
   }
 
   @override
@@ -267,9 +268,7 @@ class ManifestAssetBundle implements AssetBundle {
     if (!_splitDeferredAssets || !deferredComponentsEnabled) {
       // Include the assets in the regular set of assets if not using deferred
       // components.
-      for (final String componentName in deferredComponentsAssetVariants.keys) {
-        assetVariants.addAll(deferredComponentsAssetVariants[componentName]);
-      }
+      deferredComponentsAssetVariants.values.forEach(assetVariants.addAll);
       deferredComponentsAssetVariants.clear();
       deferredComponentsEntries.clear();
     }
@@ -457,7 +456,7 @@ class ManifestAssetBundle implements AssetBundle {
       return;
     }
     final DevFSStringContent oldContent = entries[key] as DevFSStringContent;
-    if (oldContent.string != content.string) {
+    if (oldContent?.string != content.string) {
       entries[key] = content;
     }
   }
@@ -479,8 +478,8 @@ class ManifestAssetBundle implements AssetBundle {
     // the uncompressed strings to not incur decompression/decoding while making
     // the comparison.
     if (!entries.containsKey(_kNoticeZippedFile) ||
-        !(entries[_kNoticeZippedFile] as DevFSStringCompressingBytesContent)
-            .equals(combinedLicenses)) {
+        (entries[_kNoticeZippedFile] as DevFSStringCompressingBytesContent)
+            ?.equals(combinedLicenses) != true) {
       entries[_kNoticeZippedFile] = DevFSStringCompressingBytesContent(
         combinedLicenses,
         // A zlib dictionary is a hinting string sequence with the most
@@ -494,8 +493,16 @@ class ManifestAssetBundle implements AssetBundle {
   List<_Asset> _getMaterialAssets() {
     final List<_Asset> result = <_Asset>[];
     for (final Map<String, Object> family in kMaterialFonts) {
-      for (final Map<String, Object> font in family['fonts'] as List<Map<String, Object>>) {
-        final Uri entryUri = _fileSystem.path.toUri(font['asset'] as String);
+      final Object fonts = family['fonts'];
+      if (fonts == null) {
+        continue;
+      }
+      for (final Map<String, Object> font in fonts as List<Map<String, String>>) {
+        final String asset = font['asset'] as String;
+        if (asset == null) {
+          continue;
+        }
+        final Uri entryUri = _fileSystem.path.toUri(asset);
         result.add(_Asset(
           baseDir: _fileSystem.path.join(Cache.flutterRoot, 'bin', 'cache', 'artifacts', 'material_fonts'),
           relativeUri: Uri(path: entryUri.pathSegments.last),
@@ -577,22 +584,21 @@ class ManifestAssetBundle implements AssetBundle {
     Map<String, Map<_Asset, List<_Asset>>> deferredComponentsAssetVariants
   ) {
     final Map<String, List<String>> jsonObject = <String, List<String>>{};
-    final List<_Asset> assets = assetVariants.keys.toList();
     final Map<_Asset, List<String>> jsonEntries = <_Asset, List<String>>{};
-    for (final _Asset main in assets) {
+    assetVariants.forEach((_Asset main, List<_Asset> variants) {
       jsonEntries[main] = <String>[
-        for (final _Asset variant in assetVariants[main])
+        for (final _Asset variant in variants)
           variant.entryUri.path,
       ];
-    }
+    });
     if (deferredComponentsAssetVariants != null) {
       for (final Map<_Asset, List<_Asset>> componentAssets in deferredComponentsAssetVariants.values) {
-        for (final _Asset main in componentAssets.keys) {
+        componentAssets.forEach((_Asset main, List<_Asset> variants) {
           jsonEntries[main] = <String>[
-            for (final _Asset variant in componentAssets[main])
+            for (final _Asset variant in variants)
               variant.entryUri.path,
           ];
-        }
+        });
       }
     }
     final List<_Asset> sortedKeys = jsonEntries.keys.toList()
@@ -617,7 +623,7 @@ class ManifestAssetBundle implements AssetBundle {
         final Uri assetUri = fontAsset.assetUri;
         if (assetUri.pathSegments.first == 'packages' &&
             !_fileSystem.isFileSync(_fileSystem.path.fromUri(
-              packageConfig[packageName].packageUriRoot.resolve('../${assetUri.path}')))) {
+              packageConfig[packageName]?.packageUriRoot?.resolve('../${assetUri.path}')))) {
           packageFontAssets.add(FontAsset(
             fontAsset.assetUri,
             weight: fontAsset.weight,
@@ -788,16 +794,17 @@ class ManifestAssetBundle implements AssetBundle {
       final Uri relativeUri = _fileSystem.path.toUri(relativePath);
       final Uri entryUri = asset.symbolicPrefixUri == null
           ? relativeUri
-          : asset.symbolicPrefixUri.resolveUri(relativeUri);
-
-      variants.add(
-        _Asset(
-          baseDir: asset.baseDir,
-          entryUri: entryUri,
-          relativeUri: relativeUri,
-          package: attributedPackage,
-        ),
-      );
+          : asset.symbolicPrefixUri?.resolveUri(relativeUri);
+      if (entryUri != null) {
+        variants.add(
+          _Asset(
+            baseDir: asset.baseDir,
+            entryUri: entryUri,
+            relativeUri: relativeUri,
+            package: attributedPackage,
+          ),
+        );
+      }
     }
 
     result[asset] = variants;
@@ -862,9 +869,9 @@ class ManifestAssetBundle implements AssetBundle {
 @immutable
 class _Asset {
   const _Asset({
-    this.baseDir,
-    this.relativeUri,
-    this.entryUri,
+    @required this.baseDir,
+    @required this.relativeUri,
+    @required this.entryUri,
     @required this.package,
   });
 
diff --git a/packages/flutter_tools/lib/src/devfs.dart b/packages/flutter_tools/lib/src/devfs.dart
index adcb252dd8..e005aaa2a2 100644
--- a/packages/flutter_tools/lib/src/devfs.dart
+++ b/packages/flutter_tools/lib/src/devfs.dart
@@ -64,8 +64,9 @@ class DevFSFileContent extends DevFSContent {
   FileStat _fileStat;
 
   File _getFile() {
-    if (_linkTarget != null) {
-      return _linkTarget;
+    final File linkTarget = _linkTarget;
+    if (linkTarget != null) {
+      return linkTarget;
     }
     if (file is Link) {
       // The link target.
@@ -75,9 +76,10 @@ class DevFSFileContent extends DevFSContent {
   }
 
   void _stat() {
-    if (_linkTarget != null) {
+    final File linkTarget = _linkTarget;
+    if (linkTarget != null) {
       // Stat the cached symlink target.
-      final FileStat fileStat = _linkTarget.statSync();
+      final FileStat fileStat = linkTarget.statSync();
       if (fileStat.type == FileSystemEntityType.notFound) {
         _linkTarget = null;
       } else {
@@ -87,7 +89,7 @@ class DevFSFileContent extends DevFSContent {
     }
     final FileStat fileStat = file.statSync();
     _fileStat = fileStat.type == FileSystemEntityType.notFound ? null : fileStat;
-    if (_fileStat != null && _fileStat.type == FileSystemEntityType.link) {
+    if (_fileStat != null && _fileStat?.type == FileSystemEntityType.link) {
       // Resolve, stat, and maybe cache the symlink target.
       final String resolved = file.resolveSymbolicLinksSync();
       final File linkTarget = file.fileSystem.file(resolved);
@@ -96,7 +98,7 @@ class DevFSFileContent extends DevFSContent {
       if (fileStat.type == FileSystemEntityType.notFound) {
         _fileStat = null;
         _linkTarget = null;
-      } else if (devFSConfig.cacheSymlinks) {
+      } else if (devFSConfig?.cacheSymlinks == true) {
         _linkTarget = linkTarget;
       }
     }
@@ -104,25 +106,27 @@ class DevFSFileContent extends DevFSContent {
 
   @override
   bool get isModified {
-    final FileStat _oldFileStat = _fileStat;
+    final FileStat oldFileStat = _fileStat;
     _stat();
-    if (_oldFileStat == null && _fileStat == null) {
+    final FileStat newFileStat = _fileStat;
+    if (oldFileStat == null && newFileStat == null) {
       return false;
     }
-    return _oldFileStat == null || _fileStat == null || _fileStat.modified.isAfter(_oldFileStat.modified);
+    return oldFileStat == null || newFileStat == null || newFileStat.modified.isAfter(oldFileStat.modified);
   }
 
   @override
   bool isModifiedAfter(DateTime time) {
-    final FileStat _oldFileStat = _fileStat;
+    final FileStat oldFileStat = _fileStat;
     _stat();
-    if (_oldFileStat == null && _fileStat == null) {
+    final FileStat newFileStat = _fileStat;
+    if (oldFileStat == null && newFileStat == null) {
       return false;
     }
     return time == null
-        || _oldFileStat == null
-        || _fileStat == null
-        || _fileStat.modified.isAfter(time);
+        || oldFileStat == null
+        || newFileStat == null
+        || newFileStat.modified.isAfter(time);
   }
 
   @override
@@ -135,7 +139,7 @@ class DevFSFileContent extends DevFSContent {
   }
 
   @override
-  Future<List<int>> contentsAsBytes() => _getFile().readAsBytes();
+  Future<List<int>> contentsAsBytes() async => _getFile().readAsBytes();
 
   @override
   Stream<List<int>> contentsAsStream() => _getFile().openRead();
@@ -308,8 +312,8 @@ class _DevFSHttpWriter implements DevFSWriter {
   static const int kMaxInFlight = 3;
 
   int _inFlight = 0;
-  Map<Uri, DevFSContent> _outstanding;
-  Completer<void> _completer;
+  Map<Uri, DevFSContent> _outstanding = <Uri, DevFSContent>{};
+  Completer<void> _completer = Completer<void>();
 
   @override
   Future<void> write(Map<Uri, DevFSContent> entries, Uri devFSBase, [DevFSWriter parent]) async {
@@ -553,8 +557,9 @@ class DevFS {
   /// If any other changes were made, or there is an error scanning the file,
   /// return `null`.
   String _checkIfSingleWidgetReloadApplied() {
-    if (_widgetCacheOutputFile != null && _widgetCacheOutputFile.existsSync()) {
-      final String widget = _widgetCacheOutputFile.readAsStringSync().trim();
+    final File widgetCacheOutputFile = _widgetCacheOutputFile;
+    if (widgetCacheOutputFile != null && widgetCacheOutputFile.existsSync()) {
+      final String widget = widgetCacheOutputFile.readAsStringSync().trim();
       if (widget.isNotEmpty) {
         return widget;
       }