Add ToPrimitive method priority testing

test/built-ins/String/prototype/trimStart/this-value-toprimitive-meth-priority.js

@@ -0,0 +1,95 @@
+// Copyright (C) 2017 the Valerie Young. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-String.prototype.trimStart
+description: >
+    ToString perfers Symbol.toPrimitive to toString to valueOf
+info: |
+  Runtime Semantics: TrimString ( string, where )
+  1. Let str be ? RequireObjectCoercible(string).
+  2. Let S be ? ToString(str).
+   ...
+
+  ToString ( argument )
+  If arguement is Object:
+    1. Let primValue be ? ToPrimitive(argument, hint String).
+   ...
+
+  ToPrimitive ( input [, PreferredType ])
+   ...
+    b. Else if PreferredType is hint String, let hint be "string".
+   ...
+    d. Let exoticToPrim be ? GetMethod(input, @@toPrimitive)
+    e. If exoticToPrim is not undefined, then
+      i. Let result be ? Call(exoticToPrim, input, « hint »).
+      ii. If Type(result) is not Object, return result.
+      iii. Throw a TypeError exception.
+    f. If hint is "default", set hint to "number".
+    g. Return ? OrdinaryToPrimitive(input, hint).
+   ...
+
+   OrdinaryToPrimitive( O, hint )
+    ...
+     3. If hint is "string", then
+       a. Let methodNames be « "toString", "valueOf" ».
+    ...
+features: [string-trimming, Symbol.toPrimitive]
+---*/
+
+var trimStart = String.prototype.trimStart;
+
+var called = 0;
+var thisVal = {
+  get [Symbol.toPrimitive]() {
+    called += 1;
+    return function() { return '' };
+  },
+  toString: function() {
+    throw new Test262Error(
+	'this.toString called before this[Symbol.toPrimitive]'
+    );
+  },
+  valueOf: function() {
+    throw new Test262Error(
+	'this.valueOf called before this[Symbol.toPrimitive]'
+    );
+  },
+};
+
+// Test that thisVal[Symbol.toPrimitive] is called before toString or valueOf
+trimStart.call(thisVal);
+assert.sameValue(called, 1, '[Symbol.toPrimitive] expected to have been called');
+
+var called = 0;
+var thisVal = {
+  [Symbol.toPrimitive]: undefined,
+  get toString() {
+    called += 1;
+    return function() { return '' };
+  },
+  valueOf: function() {
+    throw new Test262Error(
+      'this.valueOf called before this[Symbol.toPrimitive]'
+    );
+  },
+};
+
+// Test that toString is called before valueOf
+trimStart.call(thisVal);
+assert.sameValue(called, 1, 'this.toString expected to have been called');
+
+var called = 0;
+var thisVal = {
+  [Symbol.toPrimitive]: undefined,
+  toString: undefined,
+  get valueOf() {
+    called += 1;
+    return function() { return '' };
+  },
+};
+
+// Test that valueOf is called when neither [Symbol.toPrimitive] nor toString
+//  are defined.
+trimStart.call(thisVal);
+assert.sameValue(called, 1, 'this.valueOf expected to have been called');