Add support for assert statements with messages to the analyzer.

pkg/analyzer/lib/src/generated/error.dart

Index: pkg/analyzer/lib/src/generated/error.dart
diff --git a/pkg/analyzer/lib/src/generated/error.dart b/pkg/analyzer/lib/src/generated/error.dart
index f547807a830a9f9faddff998a7db1c78655414e4..61a6c75beeece4ec4e26e2a2cc0fd966dbdd8793 100644
--- a/pkg/analyzer/lib/src/generated/error.dart
+++ b/pkg/analyzer/lib/src/generated/error.dart
@@ -1178,6 +1178,15 @@ class CompileTimeErrorCode extends ErrorCode {
           "This class cannot extend the deferred class '{0}'");
 
   /**
+   * DEP 37 extends the syntax for assert() to allow a second "message"
+   * argument.  We issue this error if the user tries to supply a "message"
+   * argument but the DEP is not enabled.
+   */
+  static const CompileTimeErrorCode EXTRA_ARGUMENT_TO_ASSERT =
+      const CompileTimeErrorCode('EXTRA_ARGUMENT_TO_ASSERT',
+          "Assertions only accept a single argument");
+
+  /**
    * 12.14.2 Binding Actuals to Formals: It is a static warning if <i>m &lt;
    * h</i> or if <i>m &gt; n</i>.
    *