Better hints for await in non-async functions

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 a9cad07c86704eb3d68b3dc82df16ed1d4dbfa4f..43c4efad5f9c49db6c42a294352f0903e43e3e27 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -121,6 +121,22 @@ class MessageKind {
   static const MessageKind MEMBER_NOT_FOUND = const MessageKind(
       "No member named '#{memberName}' in class '#{className}'.");
 
+  static const MessageKind AWAIT_MEMBER_NOT_FOUND = const MessageKind(
+      "No member named 'await' in class '#{className}'.",
+      howToFix: "Did you mean to add the 'async' marker "
+                "to #{enclosingFunctionText}?",
+      examples: const ["""
+class A {
+  m() => await -3;
+}
+main() => new A().m();
+""", """
+class A {
+  m() => () => await -3;
+}
+main() => new A().m();
+"""]);
+
   static const MessageKind METHOD_NOT_FOUND = const MessageKind(
       "No method named '#{memberName}' in class '#{className}'.");
 
@@ -155,6 +171,16 @@ class MessageKind {
   static const MessageKind CANNOT_RESOLVE = const MessageKind(
       "Cannot resolve '#{name}'.");
 
+  static const MessageKind CANNOT_RESOLVE_AWAIT = const MessageKind(
+      "Cannot resolve '#{name}'.",
+      howToFix: "Did you mean to add the 'async' marker "
+                "to #{enclosingFunctionText}?",
+      examples: const [
+          "main() => await -3;",
+          "main() { (() => await -3)(); }",
+          "foo() => await -3; main() => foo();"
+      ]);
+
   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.",