Avoid using Invalid{Statement,Expression,Initializer}.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index fc6476d6a6edce75691183cfd8dd7f80a504044f..925d2ec01e97661ee2466be2dcacbfe9df6fd7c4 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -24,10 +24,13 @@ import 'package:front_end/src/fasta/type_inference/type_inferrer.dart';
 import 'package:front_end/src/fasta/type_inference/type_promotion.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_elimination.dart';
-import 'package:kernel/ast.dart';
+import 'package:kernel/ast.dart'
+    hide InvalidExpression, InvalidInitializer, InvalidStatement;
 import 'package:kernel/frontend/accessors.dart';
 import 'package:kernel/type_algebra.dart';
 
+import '../errors.dart' show internalError;
+
 /// Computes the return type of a (possibly factory) constructor.
 InterfaceType computeConstructorReturnType(Member constructor) {
   if (constructor is Constructor) {
@@ -181,7 +184,7 @@ class KernelCascadeExpression extends Let implements KernelExpression {
   KernelCascadeExpression(KernelVariableDeclaration variable)
       : super(
             variable,
-            makeLet(new VariableDeclaration.forValue(new InvalidExpression()),
+            makeLet(new VariableDeclaration.forValue(new _UnfinishedCascade()),
                 new VariableGet(variable))) {
     nextCascade = body;
   }
@@ -189,9 +192,9 @@ class KernelCascadeExpression extends Let implements KernelExpression {
   /// Adds a new unfinalized section to the end of the cascade.  Should be
   /// called after the previous cascade section has been finalized.
   void extend() {
-    assert(nextCascade.variable.initializer is! InvalidExpression);
+    assert(nextCascade.variable.initializer is! _UnfinishedCascade);
     Let newCascade = makeLet(
-        new VariableDeclaration.forValue(new InvalidExpression()),
+        new VariableDeclaration.forValue(new _UnfinishedCascade()),
         nextCascade.body);
     nextCascade.body = newCascade;
     newCascade.parent = nextCascade;
@@ -200,7 +203,7 @@ class KernelCascadeExpression extends Let implements KernelExpression {
 
   /// Finalizes the last cascade section with the given [expression].
   void finalize(Expression expression) {
-    assert(nextCascade.variable.initializer is InvalidExpression);
+    assert(nextCascade.variable.initializer is _UnfinishedCascade);
     nextCascade.variable.initializer = expression;
     expression.parent = nextCascade.variable;
   }
@@ -1542,3 +1545,25 @@ class KernelYieldStatement extends YieldStatement implements KernelStatement {
     inferrer.listener.yieldStatementExit(this);
   }
 }
+
+class _UnfinishedCascade extends Expression {
+  getStaticType(types) {
+    return internalError("Internal error: Unsupported operation.");
+  }
+
+  accept(v) {
+    return internalError("Internal error: Unsupported operation.");
+  }
+
+  accept1(v, arg) {
+    return internalError("Internal error: Unsupported operation.");
+  }
+
+  visitChildren(v) {
+    return internalError("Internal error: Unsupported operation.");
+  }
+
+  transformChildren(v) {
+    return internalError("Internal error: Unsupported operation.");
+  }
+}