Throw isn't a constant expression.

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 '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
@@ skipping 1362 matching lines @@
     Expression thenExpression = popForValue();
     Expression condition = popForValue();
     push(new ConditionalExpression(
         condition, thenExpression, elseExpression, const DynamicType()));
   }
 
   @override
   void endThrowExpression(Token throwToken, Token endToken) {
     debugEvent("ThrowExpression");
     Expression expression = popForValue();
-    push(new Throw(expression)..fileOffset = throwToken.charOffset);
+    if (constantExpressionRequired) {
+      push(buildCompileTimeError(
+          "Not a constant expression.", throwToken.charOffset));
+    } else {
+      push(new Throw(expression)..fileOffset = throwToken.charOffset);
+    }
   }
 
   @override
   void endFormalParameter(Token covariantKeyword, Token thisKeyword,
       Token nameToken, FormalParameterType kind) {
     debugEvent("FormalParameter");
     // TODO(ahe): Need beginToken here.
     int charOffset = thisKeyword?.charOffset;
     if (thisKeyword != null) {
       if (!inConstructor) {
@@ skipping 1560 matching lines @@
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisAccessor) {
     return node.isSuper ? "super" : "this";
   } else if (node is FastaAccessor) {
     return node.plainNameForRead;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }