Revert "Upgrade to TypeScript 2.1, and latest grunt-ts. (#3609)"

This reverts commit 79e6881ca6.

tns-core-modules/es6.d.ts

@@ -0,0 +1,12 @@
+interface Symbol {
+    toString(): string;
+    valueOf(): Object;
+}
+
+interface SymbolConstructor {
+    prototype: Symbol;
+    (description?: string | number): symbol;
+    iterator: symbol;
+}
+
+declare var Symbol: SymbolConstructor;