Compiler gives unhelpful error messages in the presence of multiple default exports

[DanielRosenwasser]

export default class foo {

}

export default function bar() {

}

var x = 10;
export default x;

In the above example, the error messages are:

1. On foo: "Duplicate identifier 'foo'."
2. On bar: "Duplicate identifier 'bar'."
3. On export default x: "Duplicate identifier 'default'."

Then we have...

export default function foo() {

}

export default function bar() {

}

1. On foo: "Duplicate function implementation."
2. On bar: "Duplicate function implementation."

[DanielRosenwasser]

#3973 adds tests tests/cases/conformance/es6/modules/multipleDefaultExports01.ts and tests/cases/conformance/es6/modules/multipleDefaultExports02.ts.

[DanielRosenwasser]

Thanks @MartyIX!