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

tests/compiler/dart2js/compiler_helper.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.
 // Test constant folding.
 
 library compiler_helper;
 
 import "package:expect/expect.dart";
 
 import '../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart'
@@ skipping 35 matching lines @@
 
 String compile(String code, {String entry: 'main',
                              String coreSource: DEFAULT_CORELIB,
                              String interceptorsSource: DEFAULT_INTERCEPTORSLIB,
                              bool enableTypeAssertions: false,
                              bool minify: false,
                              bool analyzeAll: false}) {
   MockCompiler compiler =
       new MockCompiler(enableTypeAssertions: enableTypeAssertions,
                        coreSource: coreSource,
+                       // Type inference does not run when manually
+                       // compiling a method.
+                       disableTypeInference: true,
                        interceptorsSource: interceptorsSource,
                        enableMinification: minify);
   compiler.parseScript(code);
   lego.Element element = compiler.mainApp.find(buildSourceString(entry));
   if (element == null) return null;
   compiler.phase = Compiler.PHASE_RESOLVING;
   compiler.backend.enqueueHelpers(compiler.enqueuer.resolution,
                                   compiler.globalDependencies);
   compiler.processQueue(compiler.enqueuer.resolution, element);
   compiler.world.populate();
@@ skipping 140 matching lines @@
   final xRe = new RegExp('\\bx\\b');
   regexp = regexp.replaceAll(xRe, '(?:$anyIdentifier)');
   final spaceRe = new RegExp('\\s+');
   regexp = regexp.replaceAll(spaceRe, '(?:\\s*)');
   if (shouldMatch) {
     Expect.isTrue(new RegExp(regexp).hasMatch(generated));
   } else {
     Expect.isFalse(new RegExp(regexp).hasMatch(generated));
   }
 }