Introduce "CommonElements"

pkg/compiler/lib/src/js_backend/backend_helpers.dart

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index 5a84725c122e4965c5d73a04b3df13ced54d74a8..dc5bb125993360269a9a850b666a834355a452f1 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -347,8 +347,9 @@ class BackendHelpers {
 
     // [LinkedHashMap] is reexported from dart:collection and can therefore not
     // be loaded from dart:core in [onLibraryScanned].
-    mapLiteralClass = compiler.coreLibrary.find('LinkedHashMap');
-    assert(invariant(compiler.coreLibrary, mapLiteralClass != null,
+    mapLiteralClass = compiler.commonElements.coreLibrary.find('LinkedHashMap');
+    assert(invariant(
+        compiler.commonElements.coreLibrary, mapLiteralClass != null,
         message: "Element 'LinkedHashMap' not found in 'dart:core'."));
 
     // TODO(kasperl): Some tests do not define the special JSArray
@@ -621,7 +622,7 @@ class BackendHelpers {
   }
 
   Element get syncCompleterConstructor {
-    ClassElement classElement = find(compiler.asyncLibrary, "Completer");
+    ClassElement classElement = find(asyncLibrary, "Completer");
     classElement.ensureResolved(resolution);
     return classElement.lookupConstructor("sync");
   }
@@ -638,7 +639,7 @@ class BackendHelpers {
   }
 
   Element get streamIteratorConstructor {
-    ClassElement classElement = find(compiler.asyncLibrary, "StreamIterator");
+    ClassElement classElement = find(asyncLibrary, "StreamIterator");
     classElement.ensureResolved(resolution);
     return classElement.lookupConstructor("");
   }