fix for #520, ListMixin should work again

lib/src/compiler/compiler.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection' show HashSet;
 import 'package:args/args.dart' show ArgParser, ArgResults;
 import 'package:analyzer/analyzer.dart'
     show
         AnalysisError,
         CompilationUnit,
@@ skipping 24 matching lines @@
 /// time. (Currently there is no warm up, but there may be in the future.)
 ///
 /// The SDK source code is assumed to be immutable for the life of this class.
 ///
 /// For all other files, it is up to the [AnalysisContext] to decide whether or
 /// not any caching is performed. By default an analysis context will assume
 /// sources are immutable for the life of the context, and cache information
 /// about them.
 class ModuleCompiler {
   final AnalysisContext context;
-  final _extensionTypes = new ExtensionTypeSet();
+  final ExtensionTypeSet _extensionTypes;
 
-  ModuleCompiler.withContext(this.context);
+  ModuleCompiler.withContext(AnalysisContext context)
+      : context = context,
+        _extensionTypes = new ExtensionTypeSet(context) {
+    if (!context.analysisOptions.strongMode) {
+      throw new ArgumentError('AnalysisContext must be strong mode');
+    }
+    if (!context.sourceFactory.dartSdk.context.analysisOptions.strongMode) {
+      throw new ArgumentError('AnalysisContext must have strong mode SDK');
+    }
+  }
 
   ModuleCompiler(AnalyzerOptions analyzerOptions)
       : this.withContext(createAnalysisContextWithSources(analyzerOptions));
 
   /// Compiles a single Dart build unit into a JavaScript module.
   ///
   /// *Warning* - this may require resolving the entire world.
   /// If that is not desired, the analysis context must be pre-configured using
   /// summaries before calling this method.
   JSModuleFile compile(BuildUnit unit, CompilerOptions options) {
@@ skipping 233 matching lines @@
   }
 }
 
 /// (Public for tests) the error code used when a part is missing.
 final missingPartErrorCode = const CompileTimeErrorCode(
     'MISSING_PART', 'The part was not supplied as an input to the compiler.');
 
 /// (Public for tests) the error code used when a part is unused.
 final unusedPartWarningCode = const StaticWarningCode(
     'UNUSED_PART', 'The part was not used by any libraries being compiled.');