Add simpler reading methods to Blob interface.

This fixes #40

index.bs

@@ -65,6 +65,9 @@ spec: url
     text: url; for:/
   type: interface
     text: URL
+spec: fetch
+  type:interface
+    text:ReadableStream
 </pre>
 
 <pre class="anchors">
@@ -215,7 +218,7 @@ which must be initially set to the state of the underlying storage,
 if any such underlying storage exists.
 Further normative definition of <a>snapshot state</a> can be found for {{File}}s.
 
-<pre class="idl">
+<xmp class="idl">
 [Constructor(optional sequence&lt;BlobPart> blobParts,
              optional BlobPropertyBag options),
  Exposed=(Window,Worker), Serializable]
@@ -228,6 +231,11 @@ interface Blob {
   Blob slice(optional [Clamp] long long start,
             optional [Clamp] long long end,
             optional DOMString contentType);
+
+  // read from the Blob.
+  ReadableStream stream();
+  Promise<USVString> text();
+  Promise<ArrayBuffer> arrayBuffer();
 };
 
 enum EndingType { "transparent", "native" };
@@ -238,7 +246,7 @@ dictionary BlobPropertyBag {
 };
 
 typedef (BufferSource or Blob or USVString) BlobPart;
-</pre>
+</xmp>
 
 {{Blob}} objects are [=serializable objects=]. Their [=serialization steps=],
 given |value| and |serialized|, are:
@@ -561,6 +569,59 @@ It must act as follows:
   </pre>
 </div>
 
+<h4 id="stream-method-algo">
+The stream method</h4>
+
+The <dfn method for=Blob>stream()</dfn> method returns the contents
+of the blob as a {{ReadableStream}}.
+
+It must return the result of [=getting a stream for a Blob=] for the [=context object=].
+
+To <dfn lt="getting a stream for a Blob|getting a stream for a Blob">get a stream for a {{Blob}}</dfn>
+|blob|, run the following steps:
+
+1. Let |stream| be the result of [=construct a ReadableStream object|constructing=] a
+   {{ReadableStream}} object.
+1. Return |stream| and run the following steps [=in parallel=]:
+  1. While not all bytes of |blob| have been read:
+    1. Let |bytes| be the byte sequence that results from reading a [=chunk=] from |blob|.
+    1. If a [=file read error=] occured while reading |bytes|, [$ReadableStream/error$]
+       |stream| with a [=failure reason=] and abort these steps.
+    1. [=ReadableStream/Enqueue=] |bytes| into |stream|.
+
+    Issue: We need to specify more concretely what reading from a Blob actually does, and what
+    possible errors can happen.
+
+<h4 id="text-method-algo">
+The text method</h4>
+
+The <dfn method for=Blob>text()</dfn> method returns the contents
+of the blob as a {{USVString}}.
+
+It must act as follows:
+
+1. Let |stream| be the the result of [=getting a stream for a Blob=] for the [=context object=].
+1. Let |reader| be the result of [=getting a reader=] from |stream|.
+   If that threw an exception, return a new promise rejected with that exception.
+1. Let |promise| be the result of [=reading all bytes=] from |stream| with |reader|.
+1. Return the result of transforming |promise| by a fulfillment handler that returns the result of
+   running [=UTF-8 decode=] on its first argument.
+
+<h4 id="arraybuffer-method-algo">
+The arrayBuffer method</h4>
+
+The <dfn method for=Blob>arrayBuffer()</dfn> method returns the contents
+of the blob as an {{ArrayBuffer}}.
+
+It must act as follows:
+
+1. Let |stream| be the the result of [=getting a stream for a Blob=] for the [=context object=].
+1. Let |reader| be the result of [=getting a reader=] from |stream|.
+   If that threw an exception, return a new promise rejected with that exception.
+1. Let |promise| be the result of [=reading all bytes=] from |stream| with |reader|.
+1. Return the result of transforming |promise| by a fulfillment handler that returns
+   a new {{ArrayBuffer} whose contents are its first argument.
+
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