To what does Martin Luther King Jr. make allusions in his "I Have a Dream” speech?

King's "I Have a Dream" speech is full of literary and historical allusions. One of the most famous ones which pervades the speech is to the Declaration of Independence and the Constitution, which King describes as a "promissory note" on which the nation has "defaulted as far as her citizens of color are concerned." King tells his vast audience in Washington that they are in Washington to "cash this check." He goes on to allude to a line from Shakespeare's Richard III by describing the "sweltering summer of the Negro's discontent" (Shakespeare coined the phrase "winter of our discontent"). He alludes to the famous patriotic hymn "My Country 'tis of Thee," and to the old spiritual "Free at Last" in his stirring conclusion to the speech. Overall, the entire speech alludes to the promise of American democracy, and to the hope that the blessings of democracy will be extended to all Americans regardless of race. King also alludes to many of the struggles experienced by civil rights activists, stating that he understands many may have come from the "narrow cells" of Alabama and Mississippi to attend the march. 
https://kinginstitute.stanford.edu/king-papers/documents/i-have-dream-address-delivered-march-washington-jobs-and-freedom