jsii-diff: crashes when a method turns into a property

[eladb]

Repro:

Before:

class Foo {
  bar() { return 1; }
}

After:

class Foo {
  get bar() { return 1; }
}

Error:

TypeError: Cannot read property 'forEach' of undefined
    at compareMethod (/Users/benisrae/code/jsii/jsii/packages/jsii-diff/lib/classes-ifaces.ts:135:22)
    at Object.compareReferenceType (/Users/benisrae/code/jsii/jsii/packages/jsii-diff/lib/classes-ifaces.ts:32:7)
    at compareClasses (/Users/benisrae/code/jsii/jsii/packages/jsii-diff/lib/index.ts:31:5)
    at Object.compareAssemblies (/Users/benisrae/code/jsii/jsii/packages/jsii-diff/lib/index.ts:22:3)
    at compare (/Users/benisrae/code/jsii/jsii/packages/jsii-diff/test/util.ts:33:10)