Added a stringContaining asymmetric equality tester

* Fixes #1923.

src/core/asymmetric_equality/StringContaining.js

@@ -0,0 +1,24 @@
+getJasmineRequireObj().StringContaining = function(j$) {
+  function StringContaining(expected) {
+    if (!j$.isString_(expected)) {
+      throw new Error('Expected is not a String');
+    }
+
+    this.expected = expected;
+  }
+
+  StringContaining.prototype.asymmetricMatch = function(other) {
+    if (!j$.isString_(other)) {
+      // Arrays, etc. don't match no matter what their indexOf returns.
+      return false;
+    }
+
+    return other.indexOf(this.expected) !== -1;
+  };
+
+  StringContaining.prototype.jasmineToString = function() {
+    return '<jasmine.stringContaining("' + this.expected + '")>';
+  };
+
+  return StringContaining;
+};