Increase test coverage (#175)

* Increase test coverage

* Test for error when binding `this` in destructuring pattern

* Ignore coverage of inAsync check in parseAwait - already checked externally

* Ignore coverage of default case in checkPropClash

* Remove unused parameter isAsync from parseParenAndDistinguishExpression

* Ignore coverage of an `else` branch in flowParseTypeParameterDeclaration

* Flow: remove unused parameters to flowParseTypeAnnotatableIdentifier

* Flow: ignore coverage of pass-through throw statement in parseConditional

* Flow: Add test for error on property with type param

* Flow: ignore coverage of pass-through throw statements in parseMaybeAssign, parseArrow

* Add test for error on XML-style comment in module code

* Update test for error on method in object pattern

* Test for error: "Only '=' operator can be used for specifying default value"

test/fixtures/experimental/no-async-generators/error-without-plugin/actual.js

@@ -0,0 +1 @@
+async function * f () {}

test/fixtures/experimental/no-async-generators/error-without-plugin/options.json

@@ -0,0 +1,4 @@
+{
+  "throws": "Unexpected token (1:15)",
+  "plugins": []
+}