[clang-format] JS: don't treat is: as a type matcher

[krasimir]

Summary

Clang-format is treating all occurences of is in js as type matchers. In some
cases this is wrong, as it might be a dict key.

lib/Format/TokenAnnotator.cpp

if (Right.NestingLevel == 0 &&
       return false; // Otherwise automatic semicolon insertion would trigger.
     if (Left.is(TT_JsFatArrow) && Right.is(tok::l_brace))
       return false;
     if (Left.is(TT_JsTypeColon))
       return true;
     // Don't wrap between ":" and "!" of a strict prop init ("field!: type;").
     if (Left.is(tok::exclaim) && Right.is(tok::colon))
       return false;
-    if (Right.is(Keywords.kw_is))
+    // Look for is type annotations like:
+    // function f(): a is B { ... }
+    // Do not break before is in these cases.
+    if (Right.is(Keywords.kw_is)) {
+      const FormatToken* Next = Right.getNextNonComment();
+      // If `is` is followed by a colon, it's likely that it's a dict key, so
+      // ignore it for this check.
+      // For example this is common in Polymer:
+      // Polymer({
+      //   is: 'name',
+      //   ...
+      // });
+      if (!Next || !Next->is(tok::colon))
         return false;
+    }
     if (Left.is(Keywords.kw_in))
       return Style.BreakBeforeBinaryOperators == FormatStyle::BOS_None;
     if (Right.is(Keywords.kw_in))
       return Style.BreakBeforeBinaryOperators != FormatStyle::BOS_None;
     if (Right.is(Keywords.kw_as))
       return false; // must not break before as in 'x as type' casts
     if (Right.isOneOf(Keywords.kw_extends, Keywords.kw_infer)) {
       // extends and infer can appear as keywords in conditional types:

unittests/Format/FormatTestJS.cpp

verifyFormat("function f() {\n"
                "  return foo.bar(\n"
                "      (param): param is {\n"
                "        a: SomeType\n"
                "      }&ABC => 1)\n"
                "}",
                getGoogleJSStyleWithColumns(25));
 }
 
+TEST_F(FormatTestJS, AddsIsTheDictKeyOnNewline) {
+  // Do not confuse is, the dict key with is, the type matcher. Put is, the dict
+  // key, on a newline.
+  verifyFormat("Polymer({\n"
+               "  is: '',  //\n"
+               "  rest: 1\n"
+               "});",
+               getGoogleJSStyleWithColumns(20));
+}
+
 TEST_F(FormatTestJS, AutomaticSemicolonInsertionHeuristic) {
   verifyFormat("a\n"
                "b;",
                " a \n"
                " b ;");
   verifyFormat("a()\n"
                "b;",
                " a ()\n"