Improve File hashing logic in bundler.js.

Follow-up to #9933.

As recommended by @abernix, the sha1 hash of every file is now computed
from the file's sha512 hash, so we don't have to hash the entire contents
of the file twice with two different algorithms.

Other changes/improvements:

* Invalidate the hashes when/if `File#setContents` is called.

* Ignore `options.hash` and just compute hashes from actual file contents.
  Disagreement here would be worse than any performance benefits from
  precomputing the hash.

tools/fs/watch.js

@@ -261,19 +261,19 @@ export function readFile(absPath) {
 };
 
 export function sha1(...args) {
-  return Profile("sha1", function () {
+  return Profile.run("sha1", function () {
     var hash = createHash('sha1');
     args.forEach(arg => hash.update(arg));
     return hash.digest('hex');
-  })();
+  });
 }
 
-export function sri(...args) {
-  return Profile("sri", function () {
+export function sha512(...args) {
+  return Profile.run("sha512", function () {
     var hash = createHash('sha512');
     args.forEach(arg => hash.update(arg));
     return hash.digest('base64');
-  })();
+  });
 }
 
 export function readDirectory({absPath, include, exclude, names}) {

tools/isobuild/bundler.js

@@ -529,8 +529,6 @@ export class NodeModulesDirectory {
 // Allowed options:
 // - sourcePath: path to file on disk that will provide our contents
 // - data: contents of the file as a Buffer
-// - hash: optional, sha1 hash of the file contents, if known
-// - sri: sha512 hash of file contents in base64 encoding
 // - sourceMap: if 'data' is given, can be given instead of
 //   sourcePath. a string or a JS Object. Will be stored as Object.
 // - cacheable
@@ -599,7 +597,6 @@ class File {
     this.assets = null;
 
     this._contents = options.data || null; // contents, if known, as a Buffer
-    this._hashOfContents = options.hash || null;
     this._hash = null;
     this._sri = null;
   }
@@ -615,13 +612,9 @@ class File {
 
   hash() {
     if (! this._hash) {
-      if (! this._hashOfContents) {
-        this._hashOfContents = watch.sha1(this.contents());
-      }
-
       this._hash = watch.sha1(
         String(File._salt()),
-        this._hashOfContents,
+        this.sri(),
       );
     }
 
@@ -630,10 +623,11 @@ class File {
 
   sri() {
     if (! this._sri) {
-      this._sri = watch.sri(this.contents());
+      this._sri = watch.sha512(this.contents());
     }
+
     return this._sri;
-   }
+  }
 
   // Omit encoding to get a buffer, or provide something like 'utf8'
   // to get a string
@@ -650,12 +644,10 @@ class File {
   }
 
   setContents(b) {
-    if (!(b instanceof Buffer)) {
-      throw new Error("Must set contents to a Buffer");
-    }
+    assert.ok(Buffer.isBuffer(b), "Must pass Buffer to File#setContents");
     this._contents = b;
-    // Un-cache hash.
-    this._hashOfContents = this._hash = null;
+    // Bust the hash cache.
+    this._hash = this._sri = null;
   }
 
   size() {