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Fixes #442

By accepting this pull request when a static property name conflict is detected a new error message

TS2699: Static property 'X' conflicts with built-in property 'Function.X' of constructor function 'Y'

will be issued.

Conflicting method declarations

For example

class Foo {
    static name() {
        return "Bar";
    }
}

produces

TS2699: Static property 'name' conflicts with built-in property 'Function.name' of constructor function 'Foo'

for compile target es5 or lower. Because ES2015 runtimes seem to be able to handle potentially conflicting static method declarations natively (apart from prototype), by internally setting the native function property to writable: true, there won't be an error for target es6 in the current implementation. The absence of an error in this case just indicates, that it will be handled without a runtime error. It may still lead to unexpected results if a user is not aware of overwriting the class constructor's name property which he might want to use elsewhere to read the class name (see examples on MDN). So I leave open to discussions if we should also issue an error for these cases when targeting es6.

Conflicting property declarations

class Foo {
    static name = "Bar";
}

is impossible to fix for target es5 because of Function.name being non-configurable and non-writable by spec. and at least in some es5 runtimes. In es6 targets Function.name is configurable but non-writable by default. Without changes to the emitter above value would not be assigned. It seems like it could be made possible for es6 runtimes and above, using an emit like

Object.defineProperty(Foo, "name", {
   writable: true,
   value: "Bar"
});

Unfortunately I am not able to contribute such change myself. The pragmatic solution will be to also raise a transpile error which some may favour over a transpile, anyway.

Eventually, here's a matrix of the various static member declarations and when a transpile error will be raised:

Target es3, es5:

Target es6 and higher:

* could be enabled by changing emitter output
** property made writable internally by the runtime, once a method declaration exists

Prior work

TypeScript has been preventing the declaration of a static class member prototype since the first public commit by @mhegazy and since then issued a TS2300: Duplicate identifier 'prototype' error.

class Foo {
    static property: any;
}

There were no changes made to the existing implementation. However, having a static prototype property falls into the same category of errors as to be handled for fixing #442. Hence when running the tests for this pull request the baseline for test case tests/cases/conformance/classes/propertyMemberDeclarations/propertyNamedPrototype.ts changed and now also contains a static property conflict error. I accepted this as a new baseline.

Another attempt to fix #442 is PR GH-5396