Don't exert backpressure after the first push.

Before this commit, BaseReadableStream would immediately exert backpressure (i.e. return `false` from `push`), even if the buffer was empty at the time of being pushed into. After it, backpressure will only be exerted if the buffer is nonempty.

This sets the stage for (I think) solving #24, since before this commit, it was impossible for a BaseReadableStream to ever return `true` from `push`. After it, that is possible, as long as the stream clears its internal buffer as fast as it can.

README.md

@@ -204,6 +204,7 @@ BaseReadableStream.prototype.pipeTo = (dest, { close = true } = {}) => {
     1. Set `this.[[pulling]]` to **false**.
     1. Set `this.[[state]]` to `"readable"`.
     1. Resolve `this.[[waitPromise]]` with **undefined**.
+    1. Return **true**.
 1. If `this.[[state]]` is `"readable"`,
     1. Push `data` onto `this.[[buffer]]`.
     1. Set `this.[[pulling]]` to **false**.

reference-implementation/lib/base-readable.js

@@ -91,8 +91,6 @@ BaseReadableStream.prototype._push = function _push(data) {
   else if (this._state === 'readable') {
     this._buffer.push(data);
     this._pulling = false;
-
-    return true;
   }
 
   return false;

reference-implementation/test/base-readable-stream.js

@@ -467,3 +467,44 @@ test('BaseReadableStream cancellation puts the stream in a closed state (after w
     });
   }, t.ifError.bind(t));
 });
+
+test('BaseReadableStream returns `true` for the first `push` call; `false` thereafter, if nobody reads', function (t) {
+  t.plan(5);
+
+  var pushes = 0;
+  var stream = new BaseReadableStream({
+    start : function (push) {
+      t.equal(push('hi'), true);
+      t.equal(push('hey'), false);
+      t.equal(push('whee'), false);
+      t.equal(push('yo'), false);
+      t.equal(push('sup'), false);
+    }
+  });
+});
+
+test('BaseReadableStream continues returning `true` from `push` if the data is read out of it', function (t) {
+  t.plan(12);
+
+  var stream = new BaseReadableStream({
+    start : function (push) {
+      // Delay a bit so that the stream is successfully constructed and thus the `stream` variable references something.
+      setTimeout(function () {
+        t.equal(push('hi'), true);
+        t.equal(stream.state, 'readable');
+        t.equal(stream.read(), 'hi');
+        t.equal(stream.state, 'waiting');
+
+        t.equal(push('hey'), true);
+        t.equal(stream.state, 'readable');
+        t.equal(stream.read(), 'hey');
+        t.equal(stream.state, 'waiting');
+
+        t.equal(push('whee'), true);
+        t.equal(stream.state, 'readable');
+        t.equal(stream.read(), 'whee');
+        t.equal(stream.state, 'waiting');
+      }, 0);
+    }
+  });
+});