Add NodeUtil precedence case for ES6 new.target

This will fix the case where jscomp throws an exception whilst compiling ES6 code that uses new.target.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172368097

src/com/google/javascript/jscomp/NodeUtil.java

@@ -1612,6 +1612,7 @@ static int precedence(Token type) {
       case CALL:
       case GETELEM:
       case GETPROP:
+      case NEW_TARGET:
         // Data values
       case ARRAYLIT:
       case ARRAY_PATTERN:

test/com/google/javascript/jscomp/CodePrinterTest.java

@@ -2770,6 +2770,22 @@ public void testEs6ArrowFunctionSetsOriginalNameForArguments() {
     checkWithOriginalName(code, expectedCode, compilerOptions);
   }
 
+  public void testEs6NewTargetBare() {
+    languageMode = LanguageMode.ECMASCRIPT_2015;
+    assertPrintSame("new.target.prototype");
+  }
+
+  public void testEs6NewTargetPrototype() {
+    languageMode = LanguageMode.ECMASCRIPT_2015;
+    assertPrintSame("var callable=Object.setPrototypeOf(obj,new.target.prototype)");
+  }
+
+  public void testEs6NewTargetConditional() {
+    languageMode = LanguageMode.ECMASCRIPT_2015;
+    assertPrint("if (!new.target) throw 'Must be called with new!';",
+        "if(!new.target)throw\"Must be called with new!\";");
+  }
+
   public void testGoogScope() {
     // TODO(mknichel): Function declarations need to be rewritten to match the original source
     // instead of being assigned to a local variable with duplicate JS Doc.