clang-format: [JS] Do not collapse short interfaces.

[djasper]

Submitted as r236485.

lib/Format/UnwrappedLineFormatter.cpp

if (Tok->is(tok::r_brace) && !Tok->MustBreakBefore &&
          Tok->getNextNonComment()->is(tok::semi))) {
       // We merge empty blocks even if the line exceeds the column limit.
       Tok->SpacesRequiredBefore = 0;
       Tok->CanBreakBefore = true;
       return 1;
     } else if (Limit != 0 && Line.First->isNot(tok::kw_namespace) &&
                !startsExternCBlock(Line)) {
       // We don't merge short records.
-      if (Line.First->isOneOf(tok::kw_class, tok::kw_union, tok::kw_struct))
+      if (Line.First->isOneOf(tok::kw_class, tok::kw_union, tok::kw_struct,
+                              Keywords.kw_interface))
         return 0;
 
       // Check that we still have three lines and they fit into the limit.
       if (I + 2 == E || I[2]->Type == LT_Invalid)
         return 0;
       Limit = limitConsideringMacros(I + 2, E, Limit);
 
       if (!nextTwoLinesFitInto(I, Limit))

unittests/Format/FormatTestJS.cpp

TEST_F(FormatTestJS, ClassDeclarations) {
   verifyFormat("class C {\n  ['x' + 2]: string = 12;\n}");
   verifyFormat("class C {\n  private x: string = 12;\n}");
   verifyFormat("class C {\n  private static x: string = 12;\n}");
   verifyFormat("class C {\n  static x(): string { return 'asd'; }\n}");
   verifyFormat("class C extends P implements I {}");
   verifyFormat("class C extends p.P implements i.I {}");
 }
 
+TEST_F(FormatTestJS, InterfaceDeclarations) {
+  verifyFormat("interface I {\n"
+               "  x: string;\n"
+               "}");
+}
+
 TEST_F(FormatTestJS, MetadataAnnotations) {
   verifyFormat("@A\nclass C {\n}");
   verifyFormat("@A({arg: 'value'})\nclass C {\n}");
   verifyFormat("@A\n@B\nclass C {\n}");
   verifyFormat("class C {\n  @A x: string;\n}");
   verifyFormat("class C {\n"
                "  @A\n"
                "  private x(): string {\n"

 }
 
 TEST_F(FormatTestJS, OptionalTypes) {
   verifyFormat("function x(a?: b, c?, d?) {\n}");
   verifyFormat("class X {\n"
                "  y?: z;\n"
                "  z?;\n"
                "}");
-  verifyFormat("interface X { y?(): z; }");
+  verifyFormat("interface X {\n"
+               "  y?(): z;\n"
+               "}");
   verifyFormat("x ? 1 : 2;");
 }
 
 TEST_F(FormatTestJS, IndexSignature) {
   verifyFormat("var x: {[k: string]: v};");
 }
 
 } // end namespace tooling
 } // end namespace clang