pw_web_ui: Create mocks for testing WebSerial

Node and the browser have different stream APIs. This CL creates mocks for testing web_serial_transport, which uses the browser API.

Change-Id: Ice382bb89b10a2fa23fd4a39f9217e1d0c32a9cd
diff --git a/yarn.lock b/yarn.lock
index 50bb783..10152b9 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -691,6 +691,13 @@
   dependencies:
     glob "^7.1.3"
 
+rxjs@^6.5.5:
+  version "6.5.5"
+  resolved "https://registry.yarnpkg.com/rxjs/-/rxjs-6.5.5.tgz#c5c884e3094c8cfee31bf27eb87e54ccfc87f9ec"
+  integrity sha512-WfQI+1gohdf0Dai/Bbmk5L5ItH5tYqm3ki2c5GdWhKjalzjg93N3avFjVStyZZz+A2Em+ZxKH5bNghw9UeylGQ==
+  dependencies:
+    tslib "^1.9.0"
+
 "semver@2 || 3 || 4 || 5", semver@^5.5.0:
   version "5.7.1"
   resolved "https://registry.yarnpkg.com/semver/-/semver-5.7.1.tgz#a954f931aeba508d307bbf069eff0c01c96116f7"
@@ -832,7 +839,7 @@
     read-pkg-up "^4.0.0"
     require-main-filename "^2.0.0"
 
-tslib@^1.8.1:
+tslib@^1.8.1, tslib@^1.9.0:
   version "1.13.0"
   resolved "https://registry.yarnpkg.com/tslib/-/tslib-1.13.0.tgz#c881e13cc7015894ed914862d276436fa9a47043"
   integrity sha512-i/6DQjL8Xf3be4K/E6Wgpekn5Qasl1usyw++dAA35Ue5orEn65VIxOA+YvNNl9HV3qv70T7CNwjODHZrLwvd1Q==