Added a secondary constructor to IntervalList that takes a dict.

src/main/java/htsjdk/samtools/SAMFileHeader.java

@@ -120,6 +120,12 @@ public SAMFileHeader() {
         setAttribute(VERSION_TAG, CURRENT_VERSION);
     }
 
+    /** Constructor that initializes the sequence dictionary with the provided one. */
+    public SAMFileHeader(final SAMSequenceDictionary dict) {
+        this();
+        setSequenceDictionary(dict);
+    }
+
     public String getVersion() {
         return (String) getAttribute("VN");
     }

src/main/java/htsjdk/samtools/util/IntervalList.java

@@ -71,12 +71,15 @@
 
     /** Constructs a new interval list using the supplied header information. */
     public IntervalList(final SAMFileHeader header) {
-        if (header == null) {
-            throw new IllegalArgumentException("SAMFileHeader must be supplied.");
-        }
+        if (header == null) throw new IllegalArgumentException("SAMFileHeader must be supplied.");
         this.header = header;
     }
 
+    /** Constructs a new interval list using the supplied header information. */
+    public IntervalList(final SAMSequenceDictionary dict) {
+        this(new SAMFileHeader(dict));
+    }
+
     /** Gets the header (if there is one) for the interval list. */
     public SAMFileHeader getHeader() { return header; }
 
@@ -769,4 +772,4 @@ public int compare(final Interval lhs, final Interval rhs) {
 
         return retval;
     }
-}
+}