Split escapeJS in linker and js-envs.

And specialize them to the particular needs of each subproject:

* `java.lang.StringBuilder` versus `java.io.Writer` instead of the
  generic `java.lang.Appendable`.
* With or without tracking of how many characters are written out.

Also review and specialize the one in `ir`.

Author: sjrd

js-envs/src/main/scala/org/scalajs/jsenv/JSUtils.scala

@@ -0,0 +1,92 @@
+/*
+ * Scala.js (https://www.scala-js.org/)
+ *
+ * Copyright EPFL.
+ *
+ * Licensed under Apache License 2.0
+ * (https://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package org.scalajs.jsenv
+
+object JSUtils {
+
+  def escapeJS(str: String): String = {
+    // scalastyle:off return
+    val end = str.length
+    var i = 0
+    while (i != end) {
+      val c = str.charAt(i)
+      if (c >= 32 && c <= 126 && c != '\\' && c != '"')
+        i += 1
+      else
+        return createEscapeJSString(str)
+    }
+    str
+    // scalastyle:on return
+  }
+
+  private def createEscapeJSString(str: String): String = {
+    val sb = new java.lang.StringBuilder(2 * str.length)
+    printEscapeJS(str, sb)
+    sb.toString
+  }
+
+  /* !!! BEGIN CODE VERY SIMILAR TO ir/.../Utils.scala and
+   * linker/.../javascript/Utils.scala
+   */
+
+  private final val EscapeJSChars = "\\b\\t\\n\\v\\f\\r\\\"\\\\"
+
+  private def printEscapeJS(str: String, out: java.lang.StringBuilder): Unit = {
+    /* Note that Java and JavaScript happen to use the same encoding for
+     * Unicode, namely UTF-16, which means that 1 char from Java always equals
+     * 1 char in JavaScript. */
+    val end = str.length()
+    var i = 0
+    /* Loop prints all consecutive ASCII printable characters starting
+     * from current i and one non ASCII printable character (if it exists).
+     * The new i is set at the end of the appended characters.
+     */
+    while (i != end) {
+      val start = i
+      var c: Int = str.charAt(i)
+      // Find all consecutive ASCII printable characters from `start`
+      while (i != end && c >= 32 && c <= 126 && c != 34 && c != 92) {
+        i += 1
+        if (i != end)
+          c = str.charAt(i)
+      }
+      // Print ASCII printable characters from `start`
+      if (start != i) {
+        out.append(str, start, i)
+      }
+
+      // Print next non ASCII printable character
+      if (i != end) {
+        def escapeJSEncoded(c: Int): Unit = {
+          if (7 < c && c < 14) {
+            val i = 2 * (c - 8)
+            out.append(EscapeJSChars, i, i + 2)
+          } else if (c == 34) {
+            out.append(EscapeJSChars, 12, 14)
+          } else if (c == 92) {
+            out.append(EscapeJSChars, 14, 16)
+          } else {
+            out.append("\\u%04x".format(c))
+          }
+        }
+        escapeJSEncoded(c)
+        i += 1
+      }
+    }
+  }
+
+  /* !!! END CODE VERY SIMILAR TO ir/.../Utils.scala and
+   * linker/.../javascript/Utils.scala
+   */
+
+}

nodejs-env/src/main/scala/org/scalajs/jsenv/nodejs/NodeJSEnv.scala

@@ -18,9 +18,9 @@ import scala.annotation.tailrec
 import scala.collection.JavaConverters._
 
 import org.scalajs.jsenv._
+import org.scalajs.jsenv.JSUtils.escapeJS
 
 import org.scalajs.io._
-import org.scalajs.io.JSUtils.escapeJS
 import org.scalajs.logging._
 
 import java.io._