Allow overloading of operators in unnecessary_statements

[gaetschwartz]

Description

This PR disables the unnecessary_statements for overloaded operators, so that code like this is possible:

class Logger {
	void operator<<(String message) {/*...*/}
}

void main() {
	final l = Logger();

	l << "Hello world (づ｡◕‿‿◕｡)づ"; 
	// Unnecessary statement.
	// Try completing the statement or breaking it up. (unnecessary_statements)
}

Fixes #4468

[coveralls]

coverage: 96.812% (+0.02%) from 96.796% when pulling 0183c4d on gaetschwartz:unnecessary_statements_operator_overloading into eace82e on dart-lang:main.

[pq]

Leaving open whether we agree that we want to loosen this check (which we should settle in the associated issue), I will note that if we do, we should try and optimized these new bits.

Here's a benchmark run before this change:

And after:

Either way, it does seem likely that we can establish if an operator is overriding more directly. Maybe UnnecessaryOverrides can provide some ideas? (In particular, some variation of enclosingElement.thisType.lookUpMethod2(...)?)

[srawlins]

I don't see us accepting this, as there we are not treating #4468 as a valid issue.