Implementation of null-aware operators.

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 8977b5d9220dfd3a363dbf728a4cec8f805dcfaa..3f47f629e6fc0101931619652e3e04abe728e991 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -1175,6 +1175,15 @@ main() => new C();"""]);
   static const MessageKind ASSIGNING_TYPE = const MessageKind(
       "Cannot assign a value to a type.");
 
+  static const MessageKind IF_NULL_ASSIGNING_TYPE = const MessageKind(
+      "Cannot assign a value to a type. Note that types are never null, "
+      "so this ??= assignment has no effect.",
+      howToFix: "Try removing the '??=' assignment.",
+      options: const ['--enable-null-aware-operators'],
+      examples: const [
+          "class A {} main() { print(A ??= 3);}",
+      ]);
+
   static const MessageKind VOID_NOT_ALLOWED = const MessageKind(
       "Type 'void' can't be used here because it isn't a return type.",
       howToFix: "Try removing 'void' keyword or replace it with 'var', 'final',"
@@ -1969,6 +1978,10 @@ main() => r\"\"\"
       // This is a fall-back message that shouldn't happen.
       "Incomplete token.");
 
+  static const MessageKind NULL_AWARE_OPERATORS_DISABLED = const MessageKind(
+      "Null-aware operators like '#{operator}' are currently experimental. "
+      "You can enable them using the --enable-null-aware-operators flag.");
+
   static const MessageKind EXPONENT_MISSING = const MessageKind(
       "Numbers in exponential notation should always contain an exponent"
       " (an integer number with an optional sign).",