Implement constructor bodies and initializers in CPS->JS backend.

pkg/compiler/lib/src/js_backend/codegen/task.dart

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index 0068a2178f857b7e4829f1240591b2674e179e70..69be5209a08a1c152777a11d76b769f904b49a32 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -96,8 +96,7 @@ class CspFunctionCompiler implements FunctionCompiler {
 
   cps.FunctionDefinition compileToCpsIR(AstElement element) {
     // TODO(sigurdm): Support these constructs.
-    if (element.isGenerativeConstructorBody ||
-        element.isNative ||
+    if (element.isNative ||
         element.isField) {
       giveUp('unsupported element kind: ${element.name}:${element.kind}');
     }