Re-apply: Fix type propagation bug by always narrowing types.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 176 matching lines @@
   Node parsedTree;
 
   MockCompiler({String coreSource: DEFAULT_CORELIB,
                 String helperSource: DEFAULT_HELPERLIB,
                 String interceptorsSource: DEFAULT_INTERCEPTORSLIB,
                 String isolateHelperSource: DEFAULT_ISOLATE_HELPERLIB,
                 bool enableTypeAssertions: false,
                 bool enableMinification: false,
                 bool enableConcreteTypeInference: false,
                 int maxConcreteTypeSize: 5,
+                bool disableTypeInference: false,
                 bool analyzeAll: false,
                 bool analyzeOnly: false,
                 bool preserveComments: false})
       : warnings = [], errors = [],
         sourceFiles = new Map<String, SourceFile>(),
         super(enableTypeAssertions: enableTypeAssertions,
               enableMinification: enableMinification,
               enableConcreteTypeInference: enableConcreteTypeInference,
               maxConcreteTypeSize: maxConcreteTypeSize,
+              disableTypeInference: disableTypeInference,
               analyzeAll: analyzeAll,
               analyzeOnly: analyzeOnly,
               preserveComments: preserveComments) {
     coreLibrary = createLibrary("core", coreSource);
     // We need to set the assert method to avoid calls with a 'null'
     // target being interpreted as a call to assert.
     jsHelperLibrary = createLibrary("helper", helperSource);
     foreignLibrary = createLibrary("foreign", FOREIGN_LIBRARY);
     interceptorsLibrary = createLibrary("interceptors", interceptorsSource);
     isolateHelperLibrary = createLibrary("isolate_helper", isolateHelperSource);
@@ skipping 212 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }