dart2js cps: Fix some analyzer hints in the CPS backend.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index afc4cd763f147d8ba897303579f568d602ebfa47..ee7a8706b7f9d12dc3bc417cbaf4f3db11bc4c0a 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -3565,10 +3565,8 @@ class JsIrBuilderVisitor extends IrBuilderVisitor {
           ConstantValue interceptorValue =
               new InterceptorConstantValue(constant.representedType);
           return irBuilder.buildConstant(interceptorValue);
-        } else {
-          internalError(argument, 'expected Type as argument');
         }
-        break;
+        return internalError(argument, 'expected Type as argument');
 
       case 'JS_EFFECT':
         return irBuilder.buildNullConstant();
@@ -3649,8 +3647,7 @@ class JsIrBuilderVisitor extends IrBuilderVisitor {
             CallStructure.TWO_ARGS);
 
       default:
-        giveup(node, 'unplemented native construct: ${function.name}');
-        break;
+        return giveup(node, 'unplemented native construct: ${function.name}');
     }
   }