Compile-time errors instead of internal errors.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.body_builder;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
 import '../parser/error_kind.dart' show ErrorKind;
 
@@ skipping 2351 matching lines @@
   Expression doInvocation(int charOffset, Arguments arguments) {
     print("$uri:$charOffset: Internal error: Unhandled: ${runtimeType}");
     return internalError("Unhandled: ${runtimeType}");
   }
 
   Expression buildSimpleRead();
 
   Expression buildForEffect();
 
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
-    return internalError("not supported");
+    return makeInvalidWrite(value);
   }
 
   Expression buildNullAwareAssignment(Expression value, DartType type,
       {bool voidContext: false}) {
-    return internalError("not supported");
+    return makeInvalidWrite(value);
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {bool voidContext: false, Procedure interfaceTarget}) {
-    return internalError("not supported");
+    return makeInvalidWrite(value);
   }
 
   Expression buildPrefixIncrement(Name binaryOperator,
       {bool voidContext: false, Procedure interfaceTarget}) {
-    return internalError("not supported");
+    return makeInvalidWrite(null);
   }
 
   Expression buildPostfixIncrement(Name binaryOperator,
       {bool voidContext: false, Procedure interfaceTarget}) {
-    return internalError("not supported");
+    return makeInvalidWrite(null);
   }
 
   makeInvalidRead() => internalError("not supported");
 
-  makeInvalidWrite(Expression value) => internalError("not supported");
+  Expression makeInvalidWrite(Expression value) {
+    return helper.buildCompileTimeError(
+        "Can't be used as left-hand side of assignment.", charOffset);
+  }
 }
 
 class DelayedAssignment extends ContextAccessor {
   final Expression value;
 
   final String assignmentOperator;
 
   DelayedAssignment(BuilderHelper helper, int charOffset,
       BuilderAccessor accessor, this.value, this.assignmentOperator)
       : super(helper, charOffset, accessor);
@@ skipping 296 matching lines @@
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisAccessor) {
     return node.isSuper ? "super" : "this";
   } else if (node is BuilderAccessor) {
     return node.plainNameForRead;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }