Support passing an environment variable map to child processes.

runtime/bin/process.dart

Index: runtime/bin/process.dart
diff --git a/runtime/bin/process.dart b/runtime/bin/process.dart
index dfe9ae9da0de90196b52b7825f0581ae245ce1e9..182c39daea8412bbd291bc15840acebe47b494df 100644
--- a/runtime/bin/process.dart
+++ b/runtime/bin/process.dart
@@ -141,6 +141,16 @@ class ProcessOptions {
    * The default stderrEncoding is UTF_8.
    */
   Encoding stderrEncoding;
+
+  /**
+   * Provides the environment variables for the process. If not set
+   * the environment of the parent process is inherited.
+   *
+   * Currently, only ASCII environment variables are supported and
+   * errors are likely to occur if an environment variables with
+   * code-points outside the ASCII range is passed in.
+   */
+  Map<String, String> environment;
 }