Parse assert statements with Fasta.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index e4e9f48e0cc86b17888070951a485d1e0cceb751..9ce58150ec1bfb64d6c9235c4fe2ca72f0a7d0ec 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -579,6 +579,22 @@ class AstBuilder extends ScopeListener {
     push(ast.typeName(name, arguments)..type = cls?.rawType);
   }
 
+  @override
+  void handleAssertStatement(Token assertKeyword, Token leftParenthesis,
+      Token comma, Token rightParenthesis, Token semicolon) {
+    debugEvent("AssertStatement");
+    Expression message = popIfNotNull(comma);
+    Expression condition = pop();
+    push(ast.assertStatement(
+        toAnalyzerToken(assertKeyword),
+        toAnalyzerToken(leftParenthesis),
+        condition,
+        toAnalyzerToken(comma),
+        message,
+        toAnalyzerToken(rightParenthesis),
+        toAnalyzerToken(semicolon)));
+  }
+
   void handleAsOperator(Token operator, Token endToken) {
     debugEvent("AsOperator");
     TypeAnnotation type = pop();