Restore invariant that stack is empty in parseFunctionBody.

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

Index: pkg/analyzer/lib/src/fasta/ast_builder.dart
diff --git a/pkg/analyzer/lib/src/fasta/ast_builder.dart b/pkg/analyzer/lib/src/fasta/ast_builder.dart
index 5422a77712e42eaceadc75cdd8ba2e3d40235a01..b1364cb8bf8aa022ddc9736993b1654ea3936287 100644
--- a/pkg/analyzer/lib/src/fasta/ast_builder.dart
+++ b/pkg/analyzer/lib/src/fasta/ast_builder.dart
@@ -58,8 +58,12 @@ class AstBuilder extends ScopeListener {
   /// being parsed.
   String className;
 
+  /// If true, this is building a full AST. Otherwise, only create method
+  /// bodies.
+  final bool isFullAst;
+
   AstBuilder(this.errorReporter, this.library, this.member, this.elementStore,
-      Scope scope,
+      Scope scope, this.isFullAst,
       [Uri uri])
       : uri = uri ?? library.fileUri,
         super(scope);
@@ -403,6 +407,7 @@ class AstBuilder extends ScopeListener {
 
   void handleNoInitializers() {
     debugEvent("NoInitializers");
+    if (!isFullAst) return;
     push(NullValue.ConstructorInitializerSeparator);
     push(NullValue.ConstructorInitializers);
   }
@@ -410,6 +415,7 @@ class AstBuilder extends ScopeListener {
   void endInitializers(int count, Token beginToken, Token endToken) {
     debugEvent("Initializers");
     List<Object> initializerObjects = popList(count) ?? const [];
+    if (!isFullAst) return;
 
     push(beginToken);