Add FrontendStrategy.commonElements and remove Backend.commonElements

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 36e0986bd7690fff99548dbf9a1da545ef125d9b..aba09ba9a34dcf7a1a0eff294b12e0d752c6a71e 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -203,7 +203,7 @@ abstract class Compiler {
         : new ElementBackendStrategy(this);
     _resolution = createResolution();
     _elementEnvironment = frontendStrategy.elementEnvironment;
-    _commonElements = new CommonElements(_elementEnvironment);
+    _commonElements = frontendStrategy.commonElements;
     types = new Types(_resolution);
 
     if (options.verbose) {
@@ -428,7 +428,7 @@ abstract class Compiler {
         }
       }
     }
-    backend.onLibrariesLoaded(loadedLibraries);
+    backend.onLibrariesLoaded(frontendStrategy.commonElements, loadedLibraries);
     return loadedLibraries;
   }