j.l.Class: include componentType() in isArray() @EnsureNonNullIf annotation

[thisisalexandercook]

This PR adds the componentType() method to the @EnsureNonNull annotation expression list attached to the isArray() method. componentType() is indicated as equivalent to getComponentType() which is already included in this expression list.

[aosen-xiong]

Thanks!

[wmdietl]

Thanks!

[wmdietl]

One thing to test: componentType() was added in JDK 12.
Do we produce some warning about a missing method if we run on an older JDK?
That is, can you test whether calling isArray produces a warning on a JDK that doesn't have componentType()?

In general, we probably want warnings about contracts that refer to methods that don't exist. But for the JDK we need some special handling.
But we're not type checking older versions of the JDK, so that doesn't affect us here.

So we just need to make sure whether we get a warning on the use of a method that uses a non-existing method in a contract.

[thisisalexandercook]

Dusting the cob webs off this one:

I tested this with flag --release 11 and Class.isArray() annotated as:

@EnsuresNonNullIf(expression={"getComponentType()", "componentType()"}, result=true)

Result: calling isArray() triggers a Checker error, because componentType() does not exist in JDK 11:

/tmp/TestIsArray.java:3: error: [flowexpr.parse.error.postcondition] error parsing the postcondition expression for isArray()
    if (c.isArray()) {
                 ^
  cannot parse the expression Invalid 'componentType' because no such method
1 error