fix: build and test fixes for TS 2.1 (#13294)

modules/@angular/compiler/src/expression_parser/lexer.ts

@@ -294,6 +294,8 @@ class _Scanner {
         buffer += input.substring(marker, this.index);
         this.advance();
         let unescapedCode: number;
+        // Workaround for TS2.1-introduced type strictness
+        this.peek = this.peek;
         if (this.peek == chars.$u) {
           // 4 character hex code for unicode character.
           const hex: string = input.substring(this.index + 1, this.index + 5);