Add tests for property access w/string index sigs

Author: sandersn

tests/cases/conformance/expressions/propertyAccess/propertyAccessStringIndexSignature.ts

@@ -0,0 +1,11 @@
+interface Flags { [name: string]: boolean };
+let flags: Flags;
+flags.b;
+flags.f;
+flags.isNotNecessarilyNeverFalse;
+flags['this is fine'];
+
+interface Empty { }
+let empty: Empty;
+empty.nope;
+empty["that's ok"];

tests/cases/conformance/expressions/propertyAccess/propertyAccessStringIndexSignatureNoImplicitAny.ts

@@ -0,0 +1,12 @@
+// @noImplicitAny: true
+interface Flags { [name: string]: boolean }
+let flags: Flags;
+flags.b;
+flags.f;
+flags.isNotNecessarilyNeverFalse;
+flags['this is fine'];
+
+interface Empty { }
+let empty: Empty;
+empty.nope;
+empty["not allowed either"];