Add type inference for while and do loops.

pkg/front_end/testcases/implicit_scope_test.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class ImplicitScopeTest extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   static method alwaysTrue() → core::bool {
@@ skipping 19 matching lines @@
     for (core::int i = 0; i.{core::num::<}(10); i = i.{core::num::+}(1)) {
       dynamic a = "bar";
       dynamic b = "baz";
     }
     exp::Expect::equals("foo", a);
     exp::Expect::equals(null, b);
     do {
       dynamic a = "bar";
       dynamic b = "baz";
     }
-    while ("black".==("white"))
+    while ("black".{core::String::==}("white"))
     exp::Expect::equals("foo", a);
     exp::Expect::equals(null, b);
   }
 }
 static method main() → dynamic {
   self::ImplicitScopeTest::testMain();
 }