[Errors] _this in reserved positions

[mhegazy]

//==== tests/cases/compiler/collisionThisExpressionAndAliasInGlobal.ts (1 errors) ====
module a {
   export var b = 10;
}
var f = () => this;
import _this = a; // Error
//!!! Duplicate identifier '_this'. Compiler uses variable declaration '_this' to capture 'this' reference.

Expected: errors as above
Actual: no errors

Similar but different error:

//==== tests/cases/compiler/collisionThisExpressionAndAmbientClassInGlobal.ts (1 errors) ====
declare class _this { // no error - as no code generation
}
var f = () => this;
var a = new _this(); // Error
//!!! Expression resolves to variable declaration '_this' that compiler uses to capture 'this' reference.