Add support for assert statements with messages to the analyzer.

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index ee06547f051c669797d18aa0140af901f3bb4e4b..b0501e90ff0767cc967a1fcd9c1791c9280c1f98 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -267,6 +267,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
         (this._options.lint && !options.lint) ||
         this._options.preserveComments != options.preserveComments ||
         this._options.strongMode != options.strongMode ||
+        this._options.enableAssertMessage != options.enableAssertMessage ||
         this._options.enableStrictCallChecks !=
             options.enableStrictCallChecks ||
         this._options.enableGenericMethods != options.enableGenericMethods ||
@@ -281,6 +282,7 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     this._options.generateSdkErrors = options.generateSdkErrors;
     this._options.dart2jsHint = options.dart2jsHint;
     this._options.enableGenericMethods = options.enableGenericMethods;
+    this._options.enableAssertMessage = options.enableAssertMessage;
     this._options.enableStrictCallChecks = options.enableStrictCallChecks;
     this._options.enableSuperMixins = options.enableSuperMixins;
     this._options.hint = options.hint;