Add pageable data widget

appengine/swarming/elements/res/imp/common/pageable-data.html

+<!--
+  This in an HTML Import-able file that contains the definition
+  of the following elements:
+
+    <pageable-data>
+
+  The pageable-data element allows for easy paging of data from the swarming
+  server. It displays a button to the user to go fetch more data, which will
+  be appended to the output, rather than replaced, which is typical in
+  elements like iron-ajax.
+
+  Typical usage is
+
+      this.$.page.clear();
+      this.$.page.load("/api/frobulator?alpha=beta", this.auth_headers, 20);
+
+  which initializes the url to query and goes to fetch the first 20 records
+  from the api. The user can then click the button to get the next page of
+  data. This can be done programatically with page().
+
+  Properties:
+    // input
+    label: String, what to label the button to page for more data.
+    parse: Function, This function takes the JSON object from the server and
+        returns an array of objects. Preprocessing should be done in this
+        function, if necessary.
+
+    // output
+    busy: Boolean, if a request is in flight. Calls to page() while busy will
+        be ignored.
+    output: Array<Object> the accumulated values from the server.
+
+  Methods:
+    clear(): Reset the element, emptying output.  page() will not work until
+        a call to load() has been made.
+    load(url, headers, pageSize): Initialize the element and make a call to

[stephana, 2016/09/28 13:25:23]

empty line between the method descriptions and properties to make it easier to
read. 

[kjlubick, 2016/09/28 13:50:36]

Done.

+        the server for the first batch of data.
+    page(): Must be called after a call to load().  Fetch the next batch of
+        data from the server.
+
+  Events:
+    None.
+-->
+
+<link rel="import" href="/res/imp/common/swarming-app.html">
+
+<dom-module id="pageable-data">
+  <template>
+    <style include="swarming-app-style">
+    </style>
+
+    <button
+        on-tap="page"
+        disabled$="[[_noMore(_cursor)]]">
+      [[label]]
+    </button>
+
+  </template>
+  <script>
+  (function(){
+    var END = "END";
+    Polymer({
+      is: 'pageable-data',
+
+      properties: {
+        // input
+        label: {
+          type: String,
+          value: "Show More",
+        },
+        parse: {
+          type: Function,
+          value: function(){
+            return function(a){
+              return a;
+            };
+          },
+        },
+
+        // output
+        busy: {
+          type: Boolean,
+          value: false,
+          notify: true,
+        },
+        output: {
+          type: Array,
+          notify: true,
+        },
+
+        _cursor: {
+          type: String,
+        },
+        _url: {
+          type: String,
+        },
+        _headers: {
+          type: Object,
+        },
+        _page_size: {
+          type: Number,
+        }
+      },
+
+      clear: function() {
+        this.set("output", []);
+        this._cursor = undefined;

[stephana, 2016/09/28 13:25:23]

I would either delete the _cursor member or set it to null (preferable).
Setting it to 'undefined' IMO is a contradiction of the meaning of undefined. 

[kjlubick, 2016/09/28 13:50:36]

Done.

+      },
+
+      load: function(url, headers, pageSize) {
+        if (!url) {
+          console.log("url can't be blank");

[stephana, 2016/09/28 13:25:23]

Same as below. Assume that the client of this element uses it correctly
according to the contract you outlined in the description. 

[kjlubick, 2016/09/28 13:50:37]

Done.

+          return;
+        }
+        if (url.indexOf("?") === -1) {
+          url += "?";
+        }
+        this._url = url;
+        this._headers = headers;
+        this._page_size = pageSize || 50;
+        this.page();
+      },
+
+      page: function() {
+        if (this.busy || this._cursor === END) {
+          // ignore pages while we are loading or are at the end of the data.
+          return;
+        }
+        if (!this._url) {
+          console.log("You must first call load() before calling page()");

[stephana, 2016/09/28 13:25:23]

console.log IMO is mostly for debugging purposes and should be used sparingly in
production. Same below. 

You could throw an error, since this is a programmer's error, who is not
following the contract laid out in the description. 

You can also assume that page() is not called before the element is properly
initialized, since you stated clearly that load() needs to be called first. 

[kjlubick, 2016/09/28 13:50:36]

Done.  I made it a throw just to be safe.

+          return;
+        }
+        this.set("busy", true);
+
+        var url = this._url + "&limit=" + this._page_size;
+        if (this._cursor !== undefined) {

[stephana, 2016/09/28 13:25:23]

I would make _cursor well defined in the sense, that it's either null or a valid
value that is truthy. Initialize it to null in the very beginning and set it to
null when it's cleared. But make it so that _cursor is always has a well defined
value. 

Then this could be just 

   if (this._cursor) { ....

[kjlubick, 2016/09/28 13:50:37]

Done.

+          url += "&cursor=" + this._cursor;
+        }
+
+        sk.request("GET", url, "", this._headers).then(JSON.parse).then(function(json){
+          var vals = this.parse(json);
+          if (!this.output) {
+            this.set("output", vals);
+          } else {
+            this.push("output", ...vals);
+          }
+          this.set("_cursor", json.cursor || END);
+          this.set("busy", false);
+        }.bind(this)).catch(function(reason){
+          console.log("Reason for failure of request to " + this._url, reason);
+          this.set("busy", false);

[stephana, 2016/09/28 13:25:23]

There should be a notification of failure shown to the user other than the log
(which is usually hidden).

[kjlubick, 2016/09/28 13:50:37]

Done.

+        }.bind(this));
+      },
+
+      _noMore: function(cursor) {
+        return cursor === END;
+      }
+    });
+  })();
+  </script>
+</dom-module>