Report compile-time error if using `this` implicitly in initializers

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index 947141959a095a6b27787864827c1475335dfa7e..a9cad07c86704eb3d68b3dc82df16ed1d4dbfa4f 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -155,6 +155,18 @@ class MessageKind {
   static const MessageKind CANNOT_RESOLVE = const MessageKind(
       "Cannot resolve '#{name}'.");
 
+  static const MessageKind CANNOT_RESOLVE_IN_INITIALIZER = const MessageKind(
+      "Cannot resolve '#{name}'. It would be implicitly looked up on this "
+      "instance, but instances are not available in initializers.",
+      howToFix: "Try correcting the unresolved reference or move the "
+          "initialization to a constructor body.",
+      examples: const ["""
+class A {
+  var test = unresolvedName;
+}
+main() => new A();
+"""]);
+
   static const MessageKind CANNOT_RESOLVE_CONSTRUCTOR = const MessageKind(
       "Cannot resolve constructor '#{constructorName}'.");