Revive throw as an expression but not for rethrow.

tests/language/throw_expression_test.dart

+// Copyright (c) 2012, 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.
+
+foo(a, b, c) {}
+
+class A {
+  foo() => 42;
+}
+
+void main() {
+  Expect.throws(
+      () => foo(throw 1, 2, 3),
+      (e) => e == 1);
+  Expect.throws(
+      () => foo(1, throw 2, 3),
+      (e) => e == 2);
+  Expect.throws(
+      () => foo(1, 2, throw 3),
+      (e) => e == 3);
+  Expect.throws(
+      () => throw 3 + 7,
+      (e) => e == 10);
+  Expect.throws(
+      () => -throw 3,
+      (e) => e == 3);
+  Expect.throws(
+      () => throw -3,
+      (e) => e == -3);
+  Expect.throws(
+      () => throw 3 * 7,
+      (e) => e == 21);
+  Expect.throws(
+      () => throw new A()..foo(),
+      (e) => e is A);
+  Expect.throws(
+      ([a]) => throw ?a ? 1 : 2,
+      (e) => e == 2);
+  Expect.throws(
+      ([a]) => throw ?a,
+      (e) => e == false);
+  Expect.throws(
+      () => throw +42,
+      (e) => e == 42);
+  Expect.throws(
+      () => throw [],
+      (e) => e is List);
+  Expect.throws(
+      () => throw [42],
+      (e) => e is List);
+  Expect.throws(
+      () => throw (42),
+      (e) => e == 42);
+  Expect.throws(
+      () => throw {},
+      (e) => e is Map);
+  var value = 42;
+  Expect.throws(
+      () => throw ++value,
+      (e) => e == 43);
+  Expect.throws(
+      () => throw --value,
+      (e) => e == 42);
+
+  throw (); /// 01: compile-time error
+}