doc: don't use useless constructors in stream.md
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc, stream
Refs: http://eslint.org/docs/rules/no-useless-constructor
ES2015 provides a default class constructor if one is not specified. As such, it is unnecessary to provide an empty constructor or one that simply delegates into its parent class...
The constructors in these fragments could be completely removed to eliminate useless constructor promotion, but this would make the examples a bit bare, so I've decided to just add ellipses to hint at an extension of code.