Add an abstract base-class, which all the various Stream implementations inherit from

By having an abstract base-class, it becomes a lot clearer exactly which methods/getters are expected to exist on all Stream instances.
Furthermore, since a number of the methods are *identical* for all Stream implementations, this reduces unnecessary code duplication in the `Stream`, `DecodeStream`, and `ChunkedStream` classes.

For e.g. `gulp mozcentral`, the *built* `pdf.worker.js` files decreases from `1 619 329` to `1 616 115` bytes with this patch-series.

src/core/base_stream.js

@@ -0,0 +1,99 @@
+/* Copyright 2021 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { shadow, unreachable } from "../shared/util.js";
+
+class BaseStream {
+  constructor() {
+    if (this.constructor === BaseStream) {
+      unreachable("Cannot initialize BaseStream.");
+    }
+  }
+
+  // eslint-disable-next-line getter-return
+  get length() {
+    unreachable("Abstract getter `length` accessed");
+  }
+
+  // eslint-disable-next-line getter-return
+  get isEmpty() {
+    unreachable("Abstract getter `isEmpty` accessed");
+  }
+
+  get isDataLoaded() {
+    return shadow(this, "isDataLoaded", true);
+  }
+
+  getByte() {
+    unreachable("Abstract method `getByte` called");
+  }
+
+  getBytes(length, forceClamped = false) {
+    unreachable("Abstract method `getBytes` called");
+  }
+
+  peekByte() {
+    const peekedByte = this.getByte();
+    if (peekedByte !== -1) {
+      this.pos--;
+    }
+    return peekedByte;
+  }
+
+  peekBytes(length, forceClamped = false) {
+    const bytes = this.getBytes(length, forceClamped);
+    this.pos -= bytes.length;
+    return bytes;
+  }
+
+  getUint16() {
+    const b0 = this.getByte();
+    const b1 = this.getByte();
+    if (b0 === -1 || b1 === -1) {
+      return -1;
+    }
+    return (b0 << 8) + b1;
+  }
+
+  getInt32() {
+    const b0 = this.getByte();
+    const b1 = this.getByte();
+    const b2 = this.getByte();
+    const b3 = this.getByte();
+    return (b0 << 24) + (b1 << 16) + (b2 << 8) + b3;
+  }
+
+  getByteRange(begin, end) {
+    unreachable("Abstract method `getByteRange` called");
+  }
+
+  skip(n) {
+    this.pos += n || 1;
+  }
+
+  reset() {
+    unreachable("Abstract method `reset` called");
+  }
+
+  moveStart() {
+    unreachable("Abstract method `moveStart` called");
+  }
+
+  makeSubStream(start, length, dict = null) {
+    unreachable("Abstract method `makeSubStream` called");
+  }
+}
+
+export { BaseStream };