Add FrontendStrategy.commonElements and remove Backend.commonElements

pkg/compiler/lib/src/frontend_strategy.dart

Index: pkg/compiler/lib/src/frontend_strategy.dart
diff --git a/pkg/compiler/lib/src/frontend_strategy.dart b/pkg/compiler/lib/src/frontend_strategy.dart
index 77ab684fac1b043b77c82839096a151dcbf31120..c1950bd91602aa1899ce70bb89093bcd39f56c5a 100644
--- a/pkg/compiler/lib/src/frontend_strategy.dart
+++ b/pkg/compiler/lib/src/frontend_strategy.dart
@@ -47,6 +47,10 @@ abstract class FrontendStrategy {
   /// strategy.
   ElementEnvironment get elementEnvironment;
 
+  /// Returns the [CommonElements] for the element model used in this
+  /// strategy.
+  CommonElements get commonElements;
+
   /// Returns the [DartTypes] for the element model used in this strategy.
   DartTypes get dartTypes;