Added missing IOException to Reader.read(...) methods (#9933)

This is technically a breaking change for GWT's API, but brings it
in line with the actual JRE.

Fixes #9915

tools/api-checker/config/gwt211_212userapi.conf

@@ -149,4 +149,8 @@ excludedPackages com.google.gwt.core.client.impl\
 ##############################################
 #Api  whitelist
 # when adding to the white-list, include comments as to why the addition is
-# being made.
+# being made.
+
+# java.io.Reader missing IOException throws for read methods
+java.io.Reader::read([C) OVERRIDABLE_METHOD_EXCEPTION_TYPE_CHANGE new method has more exceptions: [Ljava/io/IOException;]
+java.io.Reader::read([CII) OVERRIDABLE_METHOD_EXCEPTION_TYPE_CHANGE new method has more exceptions: [Ljava/io/IOException;]

user/super/com/google/gwt/emul/java/io/Reader.java

@@ -59,15 +59,15 @@ public int read() throws IOException {
   /**
    * Attempts to fill {@code buf} with characters up to the size of the array.
    */
-  public int read(char[] buf) {
+  public int read(char[] buf) throws IOException {
     return read(buf, 0, buf.length);
   }
 
   /**
    * Attempts to fill {@code buf} with up to {@code len} characters. Characters
    * will be stored in {@code buf} starting at index {@code off}.
    */
-  public abstract int read(char[] buf, int off, int len);
+  public abstract int read(char[] buf, int off, int len) throws IOException;
 
   /**
    * Returns whether the stream is ready for reading characters.