feat(transpiler): Transform template strings to triple quoted Dart strings

tools/transpiler/spec/lang_spec.js

@@ -5,5 +5,10 @@ export function main() {
     it('string interpolation', function() {
       expect(`${123}-'${456}"`).toEqual('123-\'456"');
     });
+
+    it('multiline string', function () {
+      expect(`1'
+2"`).toEqual('1\'\n2"');
+    });
   });
 }