Implement the compiler API and use it in frog_leg.dart.

dart/frog/leg/compiler.dart

Index: dart/frog/leg/compiler.dart
diff --git a/dart/frog/leg/compiler.dart b/dart/frog/leg/compiler.dart
index edf0f70abc9d4880a448909c717b1d2b6ec50c19..b9544ad255ffa501d25f2eaf9d553c9e17cf099d 100644
--- a/dart/frog/leg/compiler.dart
+++ b/dart/frog/leg/compiler.dart
@@ -367,7 +367,8 @@ class Compiler implements DiagnosticListener {
   }
 
   reportWarning(Node node, var message) {}
-  reportError(Node node, var message) {}
+
+  reportError(Node node, var message) => cancel(message, node: node);
 
   Script readScript(Uri uri, [ScriptTag node]) {
     unimplemented('Compiler.readScript');